Hematopoietic stem cell transplantation (HSCT) is used to treat a variety of genetic defects and malignancies, but its usefulness is limited by pulmonary infections. Infectious complications can occur both in allogeneic and autologous transplant settings and susceptibility to infection remains elevated despite hematopoietic reconstitution. To better understand innate immune deficiencies that characterize HSCT, we developed a murine model of bacterial infection post-syngeneic (syn) bone marrow transplant (BMT). We have previously shown that these mice are more susceptible to infection with Pseudomonas aeruginosa even after the hematopoietic system is reconstituted. We identified the upregulation of cyclooxygenase-2 (COX-2) and the overproduction of PGE2 as major contributing factors to the impaired innate immune function in these mice. We identified that alveolar macrophages (AMs) and neutrophils (PMNs) had defects in innate immune functions such as phagocytosis, bacterial killing and cytokine secretion. In addition, the profile of scavenger receptors on AMs were altered post-BMT, with loss of macrophage receptor with collagenous structure (MARCO), a critical receptor for recognition of P. aeurignosa. We determined that PGE2 signaled via elevated E prostanoid 2 (EP2) receptors on these AMs to inhibit their functions. Importantly, our murine studies have shown that pharmacologic or genetic inhibition of COX-2 post-BMT restores lung innate immunity and AM function against P. aeruginosa. These results are exciting because they suggest inhibition of PGE2 signaling can be a therapeutic to improve host defense post-transplant. However, there are systemic problems with a therapeutic strategy that globally blocks all prostaglandin synthesis. Thus, one aspect of this proposal will be to test a newly developed EP2 antagonist (PF-044148948) which we have obtained from Pfizer. We believe this will be a much more specific and effective therapeutic to block the inhibitory PGE2 signaling. The application also seeks to provide insight into the following unanswered questions. 1) Why is COX-2 elevated post-BMT leading to overproduction of PGE2? 2) Do these same innate immune defects characterize allogeneic (allo) BMT? 3) Do these defects post-BMT make mice more susceptible to Gram positive infections (like Streptococcus pneumoniae) as well as Gram negative ones? 4) Can we determine whether impairment of autophagy is one mechanism for impaired killing post-BMT? 5) Are the defects we have noted in our murine model also present in human HSCT patients? Our overall hypothesis is: BMT conditioning induces transforming growth factor (TGF) secretion from lung epithelial cells. This augments miR-29b expression to block synthesis of DNA methyltransferases (DNMTs) causing hypomethylation of COX-2 leading to PGE2 overexpression in AMs. Furthermore, PGE2-EP2 induced alterations in autophagy impair host defense against P. aeruginosa and S. pneumoniae post-BMT and we speculate that host defense post-BMT can be improved via treatment with a COX inhibitor, an EP2 antagonist or via induction of autophagy. These hypotheses will be explored in the following specific aims. Aim 1) To determine if syn BMT and allo BMT mice are more susceptible to P. aeruginosa and S. pneumoniae infection and if susceptibility is related to PGE2 signaling via EP2 in mice and man Aim 2: To determine whether TGF-induced miR-29b causes COX-2 hypomethylation to increase PGE2 production post-BMT Aim 3: Aim 3: To determine if autophagy is impaired in syn and allo BMT AMs, to determine whether this is related to PGE2- EP2 signaling and the importance of autophagy to host defense post-BMT